Fuel cell stack monitoring and system control

ABSTRACT

A control method for monitoring a fuel cell stack in a fuel cell system in which the actual voltage and actual current from the fuel cell stack are monitored. A preestablished relationship between voltage and current over the operating range of the fuel cell is established. A variance value between the actual measured voltage and the expected voltage magnitude for a given actual measured current is calculated and compared with a predetermined allowable variance. An output is generated if the calculated variance value exceeds the predetermined variance. The predetermined voltage-current for the fuel cell is symbolized as a polarization curve at given operating conditions of the fuel cell. Other polarization curves may be generated and used for fuel cell stack monitoring based on different operating pressures, temperatures, hydrogen quantities.

STATEMENT OF GOVERNMENT SUPPORT

[0001] The Government of the United States of America has rights in thisinvention pursuant to Agreement No. DE-AC02-90CH10435 awarded by theU.S. Department of Energy.

CROSS REFERENCE TO CO-PENDING APPLICATION

[0002] This application discloses subject matter which is disclosed andclaimed in co-pending U.S. patent application Ser. No.______ , AttorneyDocket No. H-202971, filed in Jul., 1999 in the names of David J.Hart-Predmore and William H. Pettit, and entitled “Methanol TailgasCombustor Control Method,” the entire contents of which are incorporatedby reference.

FIELD OF THE INVENTION

[0003] This invention relates to a fuel cell system, and moreparticularly to a system having a plurality of cells which consume anH₂-rich gas to produce power for vehicle propulsion.

BACKGROUND OF THE INVENTION

[0004] Fuel cells have been used as a power source in many applications.Fuel cells have also been proposed for use in electrical vehicular powerplants to replace internal combustion engines. In proton exchangemembrane (PEM) type fuel cells, hydrogen is supplied to the anode of thefuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuelcells include a “membrane electrode assembly” (MEA) comprising a thin,proton transmissive, solid polymer membrane-electrolyte having the anodeon one of its faces and the cathode on the opposite face. The MEA issandwiched between a pair of electrically conductive elements which (1)serve as current collectors for the anode and cathode, and (2) containappropriate channels and/or openings therein for distribution the fuelcell's gaseous reactants over the surfaces of the respective anode andcathode catalysts. A plurality of individual cells are commonly bundledtogether to form a PEM fuel cell stack. The term fuel cell is typicallyused to refer to either a single cell or a plurality of cells (stack),depending on the context. A group of cells within the stack is referredto as a cluster.

[0005] In PEM fuel cells hydrogen (H₂) is the anode reactant (i.e.,fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen canbe either a pure form (O₂), or air (a mixture of O₂ and N₂). The solidpolymer electrolytes are typically made from ion exchange resins such asperfluoronated sulfonic acid. The anode/cathode typically comprisesfinely divided catalytic particles, which are often supported on carbonparticles, and admixed with a proton conductive resin. The catalyticparticles are typically costly precious metal particles. These membraneelectrode assemblies which comprise the catalyzed electrodes, arerelatively expensive to manufacture and require certain controlledconditions in order to prevent degradation thereof.

[0006] For vehicular applications, it is desirable to use a liquid fuelsuch as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g.,gasoline) as the source of hydrogen for the fuel cell. Such liquid fuelsfor the vehicle are easy to store onboard and there is a nationwideinfrastructure for supplying liquid fuels. However, such fuels must bedissociated to release the hydrogen content thereof for fueling the fuelcell. The dissociation reaction is accomplished heterogeneously within achemical fuel processor, known as a reformer, that provides thermalenergy throughout a catalyst mass and yields a reformate gas comprisingprimarily hydrogen and carbon dioxide. For example, in the steammethanol reformation process, methanol and water (as steam) are ideallyreacted to generate hydrogen and carbon dioxide according to thisreaction: CH₃OH+H₂O→CO₂+3H₂. The reforming reaction is an endothermicreaction that requires external heat for the reaction to occur.

[0007] Fuel cell systems which process a hydrocarbon fuel to produce ahydrogen-rich reformate for consumption by PEM fuel cells are known andare described in co-pending U.S. patent application Ser. Nos. 08/975,442and 08/980,087, filed in the name of William Pettit in Nov., 1997, andU.S. Ser. No. 09/187,125, Glenn W. Skala et al., filed Nov. 5, 1998, andeach assigned to General Motors Corporation, assignee of the presentinvention. A typical PEM fuel cell and its membrane electrode assembly(MEA) are described in U. S. Pat. Nos. 5,272,017 and 5,316,871, issuedrespectively Dec. 21, 1993 and May 31, 1994, and assigned to GeneralMotors Corporation, assignee of the present invention, and having asinventors Swathirajan et al.

[0008] For vehicular power plants, the reaction within the fuel cellmust be carried out under conditions which preserve the integrity of thecell and its valuable polymeric and precious metal catalyst components.Since the anode, cathode and electrolyte layers of the MEA assembly areeach formed of polymers, it is evident that such polymers may besoftened or degraded if exposed to severe operating conditions, such asan excessively high temperature. This may occur if there is a defectivecell in a stack.

[0009] Monitoring of the overall stack voltage and comparison to anominal, expected voltage for a given load or current, detects a problemafter it has occurred. Thus, it would be desirable to provide a methodand control that detects a performance decrease trend, rather than anactual problem, so that the likelihood of degradation of a fuel cell isreduced.

SUMMARY OF THE INVENTION

[0010] The present invention is a control method usable in a fuel cellsystem having a fuel cell stack wherein the hydrogen reacts with anoxidant to supply electrical power to an external load connected to thestack. The control method of the present invention comprises the stepsof:

[0011] (a) monitoring actual voltage and actual current from the fuelcell stack;

[0012] (b) determining an expected magnitude of voltage as a function ofsaid actual current based on a predetermined relationship betweenvoltage and current;

[0013] (c) calculating a variance value between said actual voltage andsaid expected voltage magnitudes; and

[0014] (d) generating a signal if said calculated variance value exceedsa predetermined variance value.

[0015] Preferably, a constant or different predetermined variance valuesare established for different loads or power output. Also, differentvariance values are established for different fuel cell stack operatingparameters.

[0016] The predetermined relationship between voltage and current for agiven fuel cell is symbolized as a voltage-current polarization curve.

[0017] The difference between the expected voltage and the measuredvoltage for a given actual current is compared with the predeterminedvariance value for the predicted voltage and/or actual current todetermine if the predetermined variance value is exceeded in either apositive or negative direction. An alarm or remedial action is taken ifthe calculated variance value exceeds the predetermined variance value.

[0018] In another aspect, the present control method also contemplatesdetermining an expected value of current as a function of the actualmeasured voltage based on the predetermined voltage-currentrelationship.

[0019] The monitoring control method of the present invention providesunique advantages in the case where a fuel cell system does not directlymonitor the rate of hydrogen flow to the fuel cell. The control methodof the present invention monitors fuel cell operation to detect whenmore power is attempted to be drawn out of the fuel cell then the fuelcell is capable of supplying where there is not enough hydrogen tocreate the desired electrical power. The control method, by providing anearly warning of such a condition, enables corrective action to beimmediately taken to prevent permanent deterioration of the fuel cellstack.

[0020] The present control method can be easily implemented in existingfuel cell controllers. Further, the present control method is usablewith any type of fuel cell.

BRIEF DESCRIPTION OF THE DRAWING

[0021] The various features, advantages and other uses of the presentinvention will become more apparent by referring to the followingdescription and drawings in which:

[0022]FIG. 1 is a flow diagram depicting a fuel cell apparatus which canutilize the fuel cell stack monitoring control method of the presentinvention;

[0023]FIG. 2 is a flow diagram of the fuel cell apparatus shown in FIG.1 connected in a pictorial representation of a use application;

[0024] FIGS. 3 is a graph depicting an exemplary fuel cell stackpolarization curve; and

[0025]FIGS. 4 and 5 are flow diagrams depicting an implementation of afuel cell stack monitoring control according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] In one aspect, the invention provides a method and system toprotect the integrity of the fuel cell stack from deterioration bydetecting significant variance from performance deemed acceptable andprovides timely opportunity to implement corrective action. The fuelcell system has a fuel processor which supplies a hydrogen-rich streamto a stack of fuel cells, wherein the hydrogen reacts with an oxidant,typically air, to supply electrical power to an external load.

[0027] In the method of the invention, the voltage and current of a fuelcell stack are monitored to detect abnormal performance relative to whatis deemed acceptable based on one or more established operatingcharacteristics for the stack. In one aspect, for every level of actualpower output from the fuel cell, there is an expected power output orrange thereof at a given load. More particularly, a fuel cell stack canbe characterized by a voltage at a given current or conversely, as acurrent at a given voltage. This is called a polarization curve. Afamily of polarization curves are determinable as a function of fuelcell stack operating conditions. These operating conditions include, butare not limited to, stack pressure, temperature, quantity of hydrogen,quantity of oxidant (O₂), nitrogen in air accompanying O₂, and quantityof CO (carbon monoxide) and other minor gases in the hydrogen-rich fuelstream. In simple terms, for a given operating condition or range ofoperating conditions, it is possible to establish a relatively nominalpolarization curve and acceptable variance from the curve, or deadband,for such operating conditions.

[0028] The relationship between current (I) and voltage (V) is able tobe established for a fuel cell stack as a function of load. This istypically an inverse proportional relationship as shown in the exemplarypolarization curve of FIG. 3. In one aspect, if the relationship betweenactual current and actual voltage is outside of the deadband or variancelimits about the curve, a diagnostic, alarm, shut down or reduced fuelcell power output is generated.

[0029] In another aspect, the relationship between V and I is predictedto vary according to a family of curves based on many stack operatingvariables. The extent to which the stack's operating variables affectactual and predicted acceptable performance varies with the designfeatures of the stack and system with which it is used. If a particularstack is sensitive to the operating conditions variables, such aspressure (P), temperature (T), feed gas flow and concentration, thenmultiple polarization curves are preferred to increase the precision ofthe diagnostic. The term polarization curve is used herein forconvenience and encompasses a family or multiple polarization curves forparticular fuel cell stack.

[0030] More specifically, the system within which the fuel cell is usedis typically subject to a wide range of operating conditions. Forexample, a high degree of variability in fuel processor-generated feedgas temperature and pressure may dictate the need for multiplepolarization curves. Thermodynamic and electrochemical phenomena withinthe stack during operation influence the level of power produced.Therefore, in one aspect, the invention contemplates establishing arelationship between voltage and current, as modified by the otheroperating variables mentioned above, for example, stack temperature andpressure. Then, variance from the relationship is calculated based onmonitored V, I, T and P. In another aspect, a relatively simplifiedimplementation is based on establishing acceptable level of variance inthe voltage-current relationship based on nominal, substantiallyconstant, temperature, pressure and other stack variables. Actual stackvoltage and current are monitored, and the variance between actual andpredicted power determined. As a result, many possible approaches areusable for comparing actual power to predicted acceptable power,depending on the complexity and number of operating variables involved.

[0031] This may be further understood with reference to the fuel cellsystem shown in FIG. 1 by example only. Therefore, before furtherdescribing the invention, it is useful to understand the system withinwhich monitoring and control of fuel cell stack operation occurs.

[0032]FIG. 1 illustrates an example of a fuel cell system. The systemmay be used in a vehicle (not shown) as an energy source for vehiclepropulsion. In the system, a hydrocarbon is processed, for example, byreformation and preferential oxidation processes to produce a reformategas which has a relatively high hydrogen content on a volume basis.Therefore, reference to hydrogen-rich or relatively high hydrogencontent, refers to such content on a volume basis which is a quantityinterchangeable with molar basis to express relative amounts ofconstituents.

[0033] The invention is hereafter described in the context of a fuelcell fueled by a reformate prepared from methanol (MeOH). However, it isto be understood that the principles embodied herein are equallyapplicable to fuel cells generally, regardless of the fuel or hydrogensource used. There are other reformable hydrocarbon andhydrogen-containing fuels such as ethanol or gasoline, which are used toproduce hydrogen.

[0034] As shown in FIG. 1, a fuel cell apparatus includes a fuelprocessor 2 for catalytically reacting methanol from a methanol stream 6and water or steam from a water stream 8 in a recirculating bed 10 and acatalytic bed 12 to form a hydrogen-rich reformate gas stream. A heatexchanger 14 is interposed between the catalytic bed 12 and apreferential oxidation (PROX) reactor 16. The reformate output gasstream comprises primarily H₂ and CO₂, but also includes N₂, CO andwater. The reformate stream passes through the preferential oxidation(PROX) reactor 16 to reduce the CO-levels therein to acceptable levels(i.e., below 20 ppm). The H₂ rich reformate 20 is then fed through valve31 into the anode chamber of a fuel cell 22. At the same time, oxygen(e.g., air) from an oxidant stream 24 is fed into the cathode chamber ofthe fuel cell 22. The hydrogen from the reformate stream 20 and theoxygen from the oxidant stream 24 react in the fuel cell 22 to produceelectricity.

[0035] Exhaust or effluent 26 from the anode side of the fuel cell 22contains some unreacted hydrogen. The exhaust or effluent 28 from thecathode side of the fuel cell 22 contains some unreacted oxygen. Air forthe oxidant stream 24 is provided by a compressor 30 and is directed tothe fuel cell 22 by a valve 32 under normal operating conditions. Duringstart-up, however, the valve 32 is actuated to provide air to the inputof a combustor 34 used to heat the fuel processor 2, as will bedescribed in more detail hereinafter.

[0036] Heat from the heat exchanger 14 heats the catalyst bed(s) 10 and12 in the fuel processor 2 and also heats the PROX 16 during start up.In this regard, the H₂O-MeOH mixture supplied to the fuel processor 2will be vaporized and preferably be recirculated/refluxed several times(e.g., 20 X) through the recirculating bed 10 in the fuel processor 2,the heat exchanger side of the bed 12, the PROX 16 and the heatexchanger 14 such that the mixture also functions as a heat transfermedium for carrying heat from the heat exchanger 14 into the beds 10 and12 of the fuel processor 2 and to the PROX 16.

[0037] The heat exchanger 14 itself is heated from exhaust gases 36exiting the catalytic combustor 34. The gases 36 exiting the heatexchanger 14 are still hot and could be passed through an expander, notshown, which could drive the compressor 30 or utilized in anothermanner. In the present implementation, as shown in FIG. 1, the exhaustgases from the fuel processor 2 pass through a regulator 38, a shutoffvalve 40 and a muffler 42 before being dumped to atmosphere.

[0038] MeOH vapor 40 emanates from a vaporizer 41 nested in the exhaustend 44 of the combustor 34. The vaporizer 41 is a heat exchanger thatextracts heat from the combustor 34 exhaust to vaporize a first fuelstream, such as liquid MeOH 46 provided to the vaporizer 41 by fuelmetering device 43 from the vehicle's fuel tank. The MeOH vapor 40exiting the vaporizer 41 and the anode effluent 26 are reacted in acatalyst section 48 of the combustor 34 lying intermediate the inlet andexhaust ends 42 and 44 respectively of the combustor 34. Oxygen isprovided to the combustor 34 either from the compressor 30 (i.e., viavalve 32) or from a second air flow stream, such as a cathode effluentstream 28 depending on system operating conditions. A valve 50 permitsdumping of the combustor exhaust 36 to atmosphere when it is not neededin the fuel processor 2.

[0039] Further details concerning the construction of the combustor 34can be had by referring to pending U.S. patent applications Ser. Nos.08/975,422 and 08/980,087 filed in the name of William Pettit in Nov.1997, the entire contents of which are incorporated herein by reference.

[0040] An electric heating element 52 is provided upstream of thecatalyst bed 48 in the combustor 34 and serves to vaporize the liquidfuel 46 entering the combustor 34, heat the gas entering the bed 48 aswell as preheating the bed 48 during start-up of the combustor 34. Theheating element 52 may or may not be catalyzed. After start-up, asdescribed hereafter, the electric heater 52 is no longer required sincethe fuel will be vaporized by the exhaust gases emanating from theexhaust end 44 of the combustor 34. A preferred electric heater 52comprises a commercially available, uncatalyzed extruded metal monolithresistance element such as is used to light off the catalyst of acatalytic converter used to treat IC engine exhaust gases.

[0041] The exhaust end 44 of the combustor 34 includes a chamber thathouses the vaporizer 41 which is a coil of metal tubing which is used tovaporize liquid fuel to fuel the combustor 34. More specifically, undernormal post-start-up conditions, air or cathode effluent 28 may beintroduced into the inlet end of the coil and mixed with liquid fuelsprayed into the inlet end via a conventional automotive type fuelinjector. The airborne atomized fuel passes through the several turns ofthe heated coil tube, and therein vaporizes and exits the tube at anoutlet which is located in the cathode effluent supply conduit. Thisvaporized first fuel stream supplements a second fuel stream or anodeeffluent 26 as fuel for the combustor 34 as may be needed to meet thetransient and steady state needs of the fuel cell apparatus. Thevaporizer coil is sized to vaporize the maximum flow rate of fuel withthe minimum combustor exhaust flow rate, and is designed to operate attemperatures exceeding the autoignition temperature of the MeOH-airmixture therein throughout its fuel operational range. Autoignitionwithin the vaporizer is avoided, however, by insuring that the velocityof the mix flowing through the coil significantly exceeds the worst-caseflame speed of the mixture which varies with the composition of theinlet streams.

[0042] The amount of heat demanded by the fuel processor 2 which is tobe supplied by the combustor 34 is dependent upon the amount of fuelinput and ultimately the desired reaction temperature in the fuelprocessor 2. To supply the heat demand of the fuel processor 2, thecombustor 34 utilizes all anode exhaust or effluent and potentially someliquid fuel. Enthalpy equations are used to determine the amount ofcathode exhaust or air to be supplied to the combustor 34 to meet thedesired temperature requirements of the combustor 34 and ultimately tosatisfy the fuel processor 2. The oxygen or air provided to thecombustor 34 includes one or both of cathode effluent exhaust 28 whichis typically a percentage of the total oxygen supplied to the cathode ofthe fuel cell 22 and a compressor output air stream depending on whetherthe apparatus is operating in a start-up mode wherein the compressor airstream is exclusively employed or in a run mode using the cathodeeffluent 28 and/or compressor air. In the run mode, any total air,oxygen or diluent demand required by the combustor 34 which is not metby the cathode effluent 28 is supplied by the compressor 30 in an amountto balance the enthalpy equations to reach the desired reactiontemperature within the combustor 34 so as to supply the amount of heatrequired by the fuel processor 2 at the desired temperature. The aircontrol is implemented via an air dilution valve 47 which is a steppermotor driven valve having a variable orifice to control the amount ofbleed-off of cathode exhaust supplied to the combustor 34.

[0043] The fuel cell apparatus operates as follows. At the beginning ofoperations when the fuel cell apparatus is cold and starting up: (1) thecompressor 30 is driven by an electric motor energized from an externalsource (e.g., a battery) to provide the necessary system air; (2) air isintroduced into the combustor 34 as well as the input end of thevaporizer 41; (3) liquid fuel 46 (e.g., MeOH) is injected into the inletend of the vaporizer 41 via a fuel injector, and atomized as finedroplets with the air flowing therein; (4) the air-MeOH droplet mixexits the vaporizer 41 and mixes with compressor air introduced into thecombustor 34, and is then introduced into the input end 42 of thecombustor 34; (5) the mix passes through a flame arrestor in the frontof the combustor 34; (6) the mix is then heated by the heater 52 tovaporize the liquid droplets and heat the mixture; (7) the preheatedvaporous mix then enters a mixing-media bed for still further intimatemixing before contacting the light-off catalyst bed; (8) upon exitingthe mixing-media bed, the mix begins oxidizing on the light-off catalystbed just before it enters a primary catalyst bed 48, or reacting sectionof the combustor 34, where substantially complete combustion of the fuelis effected; and (9) the hot exhaust gases exiting the catalyst bed areconveyed to the heat exchanger 14 associated with the fuel processor 2.

[0044] Once the fuel processor temperature has risen sufficiently toeffect and maintain the reformation process: (1) valve 32 is activatedto direct air to the cathode side of the fuel cell 22; (2) MeOH andwater are fed to the fuel processor 2 to commence the reformationreaction; (3) reformate exiting the fuel processor 2 is fed to the anodeside of the fuel cell 22; (4) anode effluent 26 from the fuel cell 22 isdirected into the combustor 34; (5) cathode effluent 28 from the fuelcell 22 is directed into the combustor 34; (6) air is introduced intothe vaporizer 41; (7) liquid methanol is sprayed into the vaporizer 41;(8) the methanol-air mix circulates through the heated vaporizer coilwhere the MeOH vaporizes; (9) the methanol-air mix along with thecathode effluent 28 then mixes with the anode effluent 26; and (10) themix is burned on the catalyst bed of the combustor 34.

[0045] During normal (i.e., post start-up) operating conditions, theheater 42 is not used as the vaporizer 41 alone vaporizes the MeOH andpreheats the MeOH-air mix. Under certain conditions, as describedhereafter, the combustor 34 could operate solely on the anode andcathode effluents, without the need for additional MeOH fuel from thevaporizer 41. Under such conditions, MeOH injection to the combustor 34is discontinued. Under other conditions, e.g., increasing power demands,supplemental fuel is provided to the combustor 34.

[0046] As described above, the combustor 34 receives multiple fuels,such as a methanol-air mix as well as anode effluent 26 from the anodeof the fuel cell 22. Oxygen depleted exhaust air 28 from the cathode ofthe fuel cell 22 and air from the compressor 30 are also supplied to thecombustor 34.

[0047] According to the present fuel cell example, a controller 150shown in FIG. 1 controls the operation of the combustor 34. Anodeexhaust or effluent plus a liquid fuel, i.e., methanol, if required,support the energy requirements of the combustor 34. An enthalpy balancemaintains the desired reaction by temperature controlling the amount ofair and/or cathode exhaust supplied to the combustor 34 to meet all fuelprocessor heat requirements.

[0048] It should be noted that the energy requirements of the apparatuscomponents are expressed herein in terms of power. This is forconvenience and is meant to express an energy rate, often in units ofkilowatts, rather than BTU per second.

[0049] The controller 150 may comprise any suitable microprocessor,microcontroller, personal computer, etc., which has central processingunit capable of executing a control program and data stored in a memory.The controller 150 may be a dedicated controller specific to thecombustor 34 or implemented in software stored in the main vehicleelectronic control module. Further, although the following descriptiondescribes a software based control program for controlling the combustor34 in various modes of operation or sequence, it will also be understoodthat the combustor control can also be implemented in part or whole bydedicated electronic circuitry.

[0050] The controller 150 controls the operation of the combustor 34 insix different modes or sequences of operation. The separate modes ofoperation include (1) combustor start-up, (2) combustor operation duringfuel processor warm-up, (3) combustor operation during fuel processorstart-up, with the fuel cell off-line, (4) combustor operation duringfuel processor run mode with the fuel cell stack on-line, and (5)combustor shutdown.

[0051] Further details concerning the construction and operation of theabove-described fuel cell apparatus can be had by referring toco-pending U.S. patent application Ser. No.______ , filed in Jun. orJul., 1999, in the names of David J. Hart-Predmore and William H.Pettit, and entitled “Methanol Tailgas Combustor Control Method”, theentire contents of which are incorporated herein by reference.

[0052] In a preferred embodiment, the fuel cell system includes the fuelcell 22 as part of an external circuit 60 (see FIG. 2) wherein a portionof the external circuit 60, comprises a battery 62, an electric motor 64and drive electronics 65 constructed and arranged to accept electricenergy from a DC/DC converter 61 coupled to the fuel cell 22 and toconvert the DC power to mechanical energy from the motor 64. The battery62 is constructed and arranged to accept and store electrical energysupplied by the fuel cell 22 and to provide electric energy to motor 64.The motor 64 is coupled to driving axle 66 to rotate wheels of a vehicle(not shown). An electrochemical engine control module (EECM) 70 and abattery pack module (BPM) 71 monitor various operating parameters,including, but not limited to, the voltage and current of the stackwhich is done by the battery pack module 71, for example. The BPM 71sends an output signal (message) to the vehicle controller 74 based onconditions monitored by the BPM 71. The vehicle controller 74 controlsoperation of the battery 62, the drive electronics 65 and the electricmotor 64 in a conventional manner.

[0053] The term “fuel cell” is often used to refer to an individual celland also may refer to a fuel cell stack which contains many individualfuel cells often on the order of one hundred or more, connected inseries. Each cell within the stack includes the membrane electrodeassembly (MEA) described earlier, and each such MEA provides itsincrement of voltage. A group of cells within the stack is referred toas a cluster.

[0054] The overall voltage of the stack or cluster voltages can bemonitored to provide a determination of its operating condition.However, this does not provide information concerning the condition ofthe stack as a function of demand (load). The electric motor 64 whichconverts electric energy from the fuel cell 22 into mechanical energyplaces a demand (load) on the fuel cell stack. By the method of theinvention, if the actual power produced by the stack is significantlydifferent from a range of power levels deemed acceptable at a givenload, a signal is generated which can activate an alarm, indicator orinitiate a fuel cell shutdown.

[0055] In a preferred embodiment, the invention provides a method forcomparing actual power to expected power or to a range of expected powerat a given load for a fuel cell stack which operates in a fuel cellsystem. In a preferred embodiment, the actual voltage and actual currentof the fuel cell stack are monitored. The expected voltage as a functionof the actual current is determined based on a predeterminedrelationship between voltage and current. In other words, apredetermined relationship between voltage and current for a particulartype of fuel cell is established, the actual current is substituted intothis relationship and an expected voltage value is then determined. Avariance between the actual voltage and the expected voltage is thencalculated. If the variance so calculated exceeds a predeterminedacceptable variance, a signal is generated and corrective action istaken or indicated. This action may include fuel cell system shutdown,partial shut down or reduced power output, etc.

[0056] The present method also is adaptable to apply the actual voltageto the voltage-current relationship and obtain an expected current atthe actual voltage.

[0057] In its simplest implementation, the variance is determined on thebasis of monitoring actual voltage and current and comparing the actualvoltage and current to the predicted acceptable relationship betweenvoltage and current as determined based on assumed conditions oftemperature, pressure and feed gas flow. The relationship betweenvoltage and current is established over the full range of expectedcurrents. Acceptable variance from values defined by voltage-currentrelationship are determined. In a more complex implementation, otherfuel cell stack operating parameters which affect performance such aspressure, temperature, supply of hydrogen-rich stream, and supply ofoxidant are also considered and may result in a plurality ofvoltage-current relationships or polarization curves for differentpressures, temperatures, hydrogen stream quantity, etc.

[0058] The variance or deadband about the polarization curve in bothpositive and negative directions with respect to the polarization curvecan be established in several different ways. First, as describedhereafter, the variance is defined as a percent error of the differencebetween the actual voltage and the expected voltage at a given loadcurrent. A three percent (3%) variance or limit can be used in thisaspect, by example. Further, as described hereafter, both low and highor negative or positive limits can be established with respect to thepolarization curve. Alternately, the absolute value of the error betweenthe expected voltage and the actual voltage can be used for variancedetermination.

[0059] It is also possible, according to another aspect of the presentinvention, to provide a non-linear deadband or variance with respect toa particular polarization curve. In this manner, in certain portions ofthe polarization curve, such as at high current levels, thepredetermined variance can be made larger or smaller than the constantvariance shown in FIG. 3.

[0060] In one preferred embodiment, the method of the present inventionutilizes polarization curve data, i.e., voltage versus current, for aselected fuel cell stack design (See FIG. 3). The polarization curve isused as a symbolic reference against which to monitor actual voltage andcurrent during operation, between start-up and shutdown. Therefore,while the fuel cell system is running, the BPM 71 measures the actualfuel cell stack current and actual voltage and compares the actualvoltage with the expected voltage on the polarization curve, which datahas been stored in memory as a two-dimensional voltage-current look-uptable. If the measured/actual voltage (FIGS. 3 and 4) is different fromthe expected voltage in the polarization curve at a given load orcurrent by a predicted variance value 75 (also stored in memory), adiagnostic is flagged and a remedial action signal is issued.

[0061] One specific implementation of the control method of the presentinvention is shown FIG. 4 in which the actual current is input to theBPM 71 or EECM 70 which accesses the polarization curve look-up table140 in memory and outputs the predicted voltage at the level of theactual current. The predicted voltage Vp is summed in step 142 as anegative value with the magnitude of the measured voltage Vm. Thedifference between Vp and Vm is divided in step 144 by the magnitude ofthe predicted voltage Vp output from the polarization curve look-uptable 140.

[0062] The result of the division is a value representing the percenterror between the measured voltage Vm and the predicted voltage Vp atthe level of the actual measured current.

[0063] The BPM 71 and/or EECM 70 implements the inventive method andcontains the necessary hardware and software for receiving inputs andcomparing the inputs to preselected values, and to carry out the methoddescribed above. If Vp is greater than Vm, the percent error is anegative value and compared with the actual negative low limit percent,such as a −3% from the above example in step 146. The resultant outputlabeled “performance low flag” is generated only when the variance orerror exceeds the negative low limit error.

[0064] Similarly, the percent error from step 144 is compared with ahigh limit expressed as a positive percent value in step 148. Only whenthe percent error (Vm is greater than Vp) exceeds the high limit will anoutput labeled “performance high flag” be generated.

[0065] In another arrangement implemented in a control program executedby the EECM 70 and the BPM 71 as per FIG. 5, the voltage and current ofthe fuel cell stack are monitored by the BPM 71 (Steps 100 and 102). Theactual current value is used by the BPM 71 to access the designpolarization curve data stored in memory. Here, predicted voltage isdetermined as a function of the actual current, Vp(I) (Step 104). Next,the predicted voltage (Vp) is compared to the actual monitored voltage(Vm) (Step 106). The difference between vp and Vm is then calculated todetermine a variance. In one arbitrary alternative, the actual monitoredvoltage (Vm) is assigned a positive value and the predicted voltage (Vp)is assigned a negative value. If the variance is negative, a signalindicating a low limit is generated (Step 108). If the variance ispositive, a signal indicating a high limit is generated (Step 110).

[0066] The fuel cell stack 22 is monitored to determine whether it is inshutdown mode (Step 112) and whether it is warm (Step 114), i.e., withina normal operating temperature range. If a shutdown mode is notrequested (Step 116) and if the voltage (low) variance exceeds anacceptable level of variance (Step 108) and if the stack is warm (Step114), then all of these conditions collectively indicate a diagnosticperformance low shutdown (Step 118). However, such low variancediagnostic shutdown is subject to a time delay 120 in Step 122 before ashutdown 124 is initiated. The time delay provides the opportunity toscreen out any noise or errant signals. Similarly, if the voltage (high)variance exceeds an acceptable limit of variance (Step 110) and if thestack is warm (Step 114) and is not already in a shutdown mode (Step112), then these conditions collectively indicate a diagnostic shutdownperformance high (Step 126). However, this high variance diagnosticshutdown is also subject to a time delay 128 in Step 130 before shutdown132 is initiated.

[0067] It should be noted that stack warm temperature condition input(114) is used to prevent diagnostic system shutdown if the system is ina start up mode. The shutdown input (112) is used to prevent adiagnostic system shutdown if the system is already in a shutdown mode.Thus, the logic of the invention is adaptable to monitor low and highpower conditions during a running mode. It is expected that a conditionof getting less power than expected is more common than the condition ofgetting more power than expected from the stack. The simplestimplementation is to include the polarization curve data in a look uptable, with a given variance (delta) fixed along the entire curve.Another option is a percent variation that changes along the curve, ordifferent variations for above and below the curve.

[0068] In the example given above, corrective action is a systemshutdown. Other actions or responses are also contemplated including analarm, indicator, reduction or power output from the fuel cell, etc.Different or the same shutdown or other remedial actions can beimplemented for a low or high shutdown.

[0069] It will be understood that FIG. 4 and the description aboverelating thereto is to be interpreted by way of an example of one way ofimplementing the control method of the present invention. Other methodssuggest themselves including:

[0070] The low limit and high limit expressed a negative percent and apositive percent of the predicted voltage can be a constant value overthe entire polarization curve or over the full range of expectedvoltages and currents. It will also be understood that the variance ordelta may be provided in varying amounts depending upon the particularlocation of the polarization curve in which the fuel cell is currentlyoperating.

[0071] The control system of the present invention is particularlyimportant where a fuel cell system does not directly monitor the rate ofhydrogen flow to the fuel cell; that is, in cases where there is not ahydrogen sensor directly upstream of the fuel cell. In a fuel cellsystem it is important to match the load being demanded of a system withthe rate at which reformate gas is supplied to the fuel cell. If it isattempted to draw more power out of the fuel cell than it is capable ofsupplying because there is not enough hydrogen to create electricalpower, then it is possible to permanently degrade the fuel cell stack.Degradation can include breakdown of the membrane, polymer components.Therefore, it is advantageous to have the enhanced detection system ofthe invention to detect the situation where the amount of actual poweris significantly different from the predicted power for a given loadpoint.

[0072] In the absence of the system of the invention, it is typical torely on absolute level of total fuel cell stack or cluster voltages,independent of the load point. This approach is useful to determine thata problem exists in the fuel cell. However, to be practical, the trippoints for the diagnostic must be set to relatively low voltages orworst case levels to avoid unnecessary diagnostics and correctiveaction. This is undesirable because a problem may arise at a voltageabove the low-voltage cut-off and the vehicle propulsion systemcontinues to drive the load whereby voltage continues to decline. Then,degradation of the cell can result and it is also possible to incur areverse polarity permanently. In this situation, the cell begins actingas a resistor and will begin heating up. As the cell continues to heatup, it will adversely affect the cell next to it and if heat effect isnot abated, further deterioration is possible. In contrast, since thepresent invention compares actual power to expected power at a givenload point, the diagnostic trip point is effectively variable as afunction of load.

[0073] The invention provides many advantages over existing alternativeswhich address high and low voltage situations. In one existingalternative, for example, at the onset of a low voltage condition onepresent strategy is to significantly increase the amount of hydrogensent to the fuel cell stack thereby increasing the amount of excesshydrogen referred to as lambda, during low voltage transients. This isless desirable since using a higher anode lambda may consume morehydrogen making the fuel cell system less efficient, precipitate needfor a larger fuel processor, and increase the cost and size of thesystem. Another option is to ignore a low voltage reading during loadtransients. This option is less desirable since the potentialconsequences to the fuel cell stack are too great. A third option, asdescribed above, is to monitor the voltage across the entire stackindependent of load and feed this back to the vehicle load controller.This option is less desirable since the fuel cell stack voltage alonemay not accurately detect a problem unless its relationship to currentat a given load point is considered.

[0074] The method of the present invention is preferred over theexisting options since, by the present invention, the fuel celldiagnostic is developed based on characteristics of the stack, actualversus expected power as a function of load, and operating conditionssuch as temperature and pressure which influence the power produced bythe stack. This latter feature is particularly useful since stackoperating temperature, nominally 70° C. to 80° C. in the laboratory, isexpected to vary considerably under seasonal conditions.

What is claimed is:
 1. In a method for operating a fuel cell systemhaving a fuel processor which supplies a hydrogen-rich stream to a stackof fuel cells, wherein said hydrogen reacts with an oxidant to supplyelectrical power to an external load, the improvement comprising: (a)monitoring actual voltage and actual current from the fuel cell stack;(b) determining an expected magnitude of voltage as a function of saidactual current based on a predetermined relationship between voltage andcurrent; (c) calculating a variance value between said actual voltageand said expected voltage magnitudes; and (d) generating a signal ifsaid calculated variance value exceeds a predetermined variance value.2. The method of claim 1 wherein before step (d), establishing differentpredetermined variance values for different loads.
 3. The method ofclaim 1 wherein before step (d), establishing different predeterminedvariance values for different fuel cell stack operating parameters. 4.The method of claim 3 wherein said different fuel cell stack operatingparameters include pressure, temperature, supply of said hydrogen-richstream and supply of said oxidant.
 5. The method of claim 1 wherein saidpredetermined relationship between voltage and current is symbolized asa polarization curve and wherein different predetermined variance valuesare established along the curve.
 6. The method of claim 1 furtherincluding terminating the supply of power to the external load.when saidpredetermined variance value is exceeded.
 7. The method of claim 1further comprising the step of establishing the predetermined variancevalue as a percentage of the expected magnitude of the voltage.
 8. Themethod of claim 7 further comprising the steps of: establishing apositive variance value as a percentage of the predicted voltage whereinthe sum of percentage and the predicted voltage magnitude are greaterthan the predicted voltage magnitude; and establishing a negativevariance value as a percentage of the predicted voltage magnitudewherein the sum of the percentage and the predicted voltage magnitude isless than the predicted voltage magnitude.
 9. The method of claim 8further comprising the step of: generating separate output signals basedon the predicted voltage magnitude exceeding the positive and thenegative variance values.
 10. In a method for operating a fuel cellsystem having a fuel processor which supplies a hydrogen-rich stream toa stack of fuel cells, wherein said hydrogen reacts with an oxidant tosupply electrical power to an external load, the improvement comprising:(a) establishing a predetermined relationship between voltage andcurrent for a fuel cell stack; (b) monitoring actual voltage and actualcurrent from the fuel cell stack; (c) then either: (1) determining anexpected value of voltage as a function of the actual current based onthe predetermined relationship; or (2) determining an expected value ofcurrent as a function of the actual voltage based on the predeterminedrelationship; (d) calculating the variance between said actual andexpected values; and (e) generating a signal if the calculated varianceexceeds a predetermined variance value.